I am building an online game, and as soon as a customer pays for "diamonds" via PayPal, I want the customer to have diamonds deposted into the player's account.

C? PHP? Python? Java?

What you have described is a complete subsystem, not a bot or a technology. That subsystem would need to include multiple parts such as user and payment database, payment gateway and/or paypal transaction processing application, payment event/status driven access controls, etc.

At this point you have an idea, but not a plan or anything with which others can really help. The language(s) chosen for implementation would depend on the specification and your available skills, neither of which are known to us.

I would suggest that you search for terms such as payment gateway and online fulfillment software as a good starting point.

Also, please review the LQ Site Faq for help in asking a complete question. The better and more complete your question, the better replies you will receive.