UPDATE November 5, 4:39am EST: This Beware has been resolved. I have received payment for my work, the client has agreed to my ToS and has taken the necessary steps to comply with my new rules regarding the artwork (with much grumbling and complaining of course; it is actually funny). For this, I no longer plan to sell the artwork; I have finally received the minimum of my original requested amount of compensation for my work, and will no longer pursue the matter.
WHO: Talen27 / Talen / Seagull / Seaguii
WHERE: FurAffinity, Discord, Telegram
WHAT: Design and reference sheet
WHEN: July 8, 2018 to October 27, 2018
EXPLAIN: Before I start, most of what happened between us happened in voice chat, so any evidence on the matter is scarce. I am fortunately able to salvage enough from our text chatlogs (that he didn't delete) that should be enough to piece together what happened .
This person commissioned me for a design for his character, along with a reference sheet. I charged him $50, and since we were lovers, I told him to hold onto it so we could put it towards a commission for us both. We fell out, I lost my desire to take any commission with him, but he refused to give me my $50 back.
To begin, Talen and I used to be in a relationship, and I was the one who introduced him to the scalie community. As a symbol of our love, I made for him, for free, a design for his character along with a reference sheet for it.
I'd like to say here that the process of creating that reference sheet for him was painful, with me streaming the process of the art to him and creating design after design that he kept asking for tweaks or redesigns on, bouncing between every little detail asking me to make drastic changes again and again. Nevertheless, I completed it and sent him the finished file. The date was July 8.
After a week of finishing the reference sheet, though, he asked for changes to the design, and along with some other personal disputes, I told him that I would charge him no less than $50, and he agreed. 
Honestly, $50 for a full reference sheet at this quality is one hell of a deal, so to this day, the fact that he is unwilling to pay up is due to pettiness.
I continued to stream to him, in the exact same process of him asking for change after change after change on the designs that I came up with. I finished up the reference sheet on July 30, and told him to hold on to the $50 so he could pay that much in my stead when we decide to go for a commission together.
Fast forward to early August when he broke up with me for other relationship-related reasons. He still wanted to be friends, and though it hurt me, I continued to be friends with him, and he was still holding on to the $50 that he owes me. 
Since then, throughout the next two months, I've made repeated mentions for him to just return me my $50, though every time he tried to convince me to just get a commission with him. He never denied it, but he knows he owes me $50. 
Fast forward again to mid-October, I made a request of him to stop mentioning all the new friends he's trying to date because I was still hurting from the breakup, and he straight up flamed me in voice chat, in the most toxic manner one can imagine, all for being "soft" or "overreacting". At that point, I snapped, and demanded my $50 back, but he refused, going between excuses such as "I can pay you the $50, but I won't" to "I only have $7 in my bank account, I can't pay you", all between a lot of name-calling, flaming, and accusing me of "blackmail" (in his technical terms, "defamation for the purposes of monetary gain", which you can totally twist "I'll tell peeps you're a thief if you steal money from me" into). He proceeded to block me from all communications. 
Upon contacting one of his friends who know him better, I learned that Talen is simply that way and that my $50 is lost.
I have contacted another of his friends recently, who willingly acted as an arbiter between me and Talen, and given him an ultimatum: pay me, or stop using my design. At this point, Talen had removed my reference sheet from his gallery, and currently plans to use one of the art commissions he got from another artist as his new reference sheet, except that commission's character was fully taken from my design and refsheet.
This one is where he suggests artists and I explain fully what I meant. In the end, it is still clear he owes me $50. I have blacked out the exact values that he had bragged about spending on commissions and toys already as well as how much he still had at the time. I have also blacked out other names and the segue into the discussion about our own personal issues.
This is our full conversation on FurAffinity directly after the last straw. Since they were all separate responses, I have taken the liberty to format the screenshots into a chatroom structure for the sake of readability; no content has been altered. Personal issues and discussions have been blacked out, except for the final one.
These are excerpts from our final argument in a separate Discord server, highlighting his denial and hypocrisy. There have been many repeated mentions of "blackmail" and threatening legal action on his part that do not contribute to the discussion, so I have cut those out. I have also blacked out what he claimed to have in his bank account; the decimal place should tell you enough.
Honestly, I was very angry at that point, so some things I said was cringe-worthy; I have left those in for the sake of context.
There are lessons to be learned from this story, in particular for me. I have learned to recognize red flags that would ruin a relationship, and I have also learned that I should never provide the final finished commission without payment first, whether I trust the person on a romantic level or not.
I understand that this may be a more "special" case of Artist Beware in that the business is intertwined with personal feud; I have tried my best to omit most of the personal stuff from my images, but some I have kept in due to their vague relation to the transaction at hand.
Another thing I am unsure of is whether I should include the artwork in question in this post for clarity and also as proof in case he actually does continue to get commissions with the same character design. I'll edit it in underneath all this if you guys want me to.
I currently plan on selling the character design and reference sheet to someone else with little to no changes to the actual design, since the design of the character is still mine.