Josh,
As someone who has had a good transaction with you (and left you good feedback) I feel compelled to let you know that you don't have a leg to stand on here.
case scenario:
You go to the electronics store and buy a Sony DVD player for $100.
The clerk gives you a box labeled "Sony DVD player".
You go home, open the box, and it's a Sony Tape player.
You take it back to the clerk to return it.
The clerk tells you, "hmm, Sony must have mislabeled this. Go ahead and try to sell this tape player".
You find a buyer for the tape player willing to pay you $50 for it, and you inform the clerk about it.
The clerk tells you, "I can give you $25 and then you'll only be out $25, and when and IF Sony pays me the remaing amount then I will give it back to you. Oh, and by the way, Sony is filing bankruptsy so your odds of recouping that remaining $25 are pretty slim".
This is not okay. While the store you bought the item from was unaware that the item was mislabeled, it is COMPLETELY their responsibility to take the item back and refund you the full amount you paid.
Your responsibility now is to either:
1. refund all the money in full FIRST and then go after your friend to reimburse you for it on your own.
2. if you cannot afford option 1, send collateral to be held until you can afford option 1.
The buyer is not being unreasonable. He simply foresees that if he accepts partial payment now, the balance will most likely never be paid.
Understand that I'm taking the time to write this because I want to help you understand the buyers perspective on this, not to make you look bad, or to attack you.
I hope you work this out without damaging your reputation further as that would be a shame, but will be inevitable should you not handle this professionally.
