Should I give my friend back his now overdue library books, despite him owing me $500 for an essay I wrote?

3.5 weeks ago a good friend offered me $500 to write a an essay for him. The deal was $300 cash on delivery and the other $200 once he receives a mark higher than 75. The mark was higher than 75. The guy has not had problems with money in recent months due to windfall. He is going away on a trip for a few weeks in the coming days. He asked if I could wait until he came back to give me the money since things were a little "tight". I told him the deal was COD, I said I want the money in one lump sump payment and ASAP. He promised 100% upon his return at the latest. He is asking for his -now overdue- library books, which I used in writing the paper. As he is a good friend, I don't doubt that he will pay me. . . eventually. The last time I lent him $1000 he took forever to pay and on top of that paid in small installments over a long period of time. SHOULD I GIVE BACK THE BOOKS TRUSTING HE WILL PAY UP, OR KEEP THEM AS COLLATERAL DESPITE RISING LIBRARY FINES (apprx. $10/day)