Before China requested for monkeys from them, they checked within their surroundings, and monkey is the only thing they can attribute value to in that entire country, so they just have to go for it. A country like Sri Lanka will take any offer given to them, especially on something that's mostly common in their locality.
China knows what they want so they extending hands in loan servicing is not bad as the loans are for a particular meaningful purpose. China is a friendly nation so I do not see it as a new thing her sister nation going to them for loan and one thing I like about China is that they do not refuse the loan, they are open and give helping hands to all countries.
Sri Lanka agreeing to pay back with monkeys is not a bad thing if that is what China opted for as loan or debt servicing, it is okay for them to get along together. Do not forget that Chinese eats animals alot and also note that animals are very strong creature and are better option for experimental analysis and test for newly produced drugs, chemicals and lots more to know how effective they can be to humans.
So therefore, I see no reasons anything wrong for such agreement if China made such request or Sri Lanka making such proposal for debt servicing.