If OP doesn't accept that TX as a loan then he must refund it immediately. There's no way it can be considered OK for him to keep it until the 16th, not to mention permanently. Obviously ManyProofs didn't mean to give it away regardless of how he wrote his message because of his lack of knowledge of English.
If I sent money by mistake to someone and he refused to refund it (minus any cost incurred by him) of course I'd consider him a scammer. This is a comparable case. I'd consider ManyProofs' actions as a mistake but of course it's not acceptable for OP to take advantage of that mistake.
The moment OP noticed that unexpected TX in his address he should have refunded immediately minus the TX fee if he didn't want to use it. Instead he spent it (getting a free gain of the coins) and now refuses to refund until the 16th as per the loan he requested (and didn't happen according to him). That's unacceptable. I hope he at least refund by then.
not saying that there shouldnt be a refund, in fact i do believe that he should get refunded, and hopefully learn from this mistake. but, i was just hoping to get the point across that
in the case there was no refund, there would be no one for manyproofs to blame but himself.
i should clarify that when i agreed with ausbit's statement, i was referring to the first half that he was lucky to be offered a refund in the first place. but at this point, as the coins sent by manyproof has already been spent, it might be easier to just consider that as the loan taken and discuss terms instead seeing as how op has offered a refund.