A client for whom we are building a travel site wishes to use PayPal for accepting online payments from customers abroad using their credit cards. But, instead of accepting the full fee for a tour, he wants to accept just an advance for registration and the remaining amount as Cash on Delivery when the tourists land up in India and are about to begin their tour.

The coding side of this is doable, but I wanted to know if PayPal has any rules against such split payments. Or, is this something PayPal won't care about? Effectively, the advance fee is the order value as far as PayPal is concerned.

it's impossible to split the payment for one service or product via PayPal.

This would be a question answered best by PayPal. You are in a sense providing non-tangible goods to the customer. PayPals buyer/seller protection does not cover non-tangible goods. Im assuming since it's an advance and not a deposit the funds will be used to pay for the service.

Since I doubt PayPal will give you an answer in writing and calling twice in a row will probably get you two different answers I would suggest the following:

Make sure the order system processes it explaining that it's an advance. So you don't want the payment for the advance being described as the actual service. The next thing is when the customer fulfills his order and completes the order have the customer complete an agreement covering the PayPal advance.

Otherwise the customer could always dispute it within 6 months and have a chance at winning chargebacks since PayPal won't be able to show that your providing anything.

Also make sure your clients site is very clear on the refund policy for the advance.