woocommerce : refund a sample at the next order [on hold]

On my site I will sell products (full size) and samples so that customers can try before buying the full size.

The idea is to sell the samples at a very cheap price, just enough to cover shipping & handling fees.

What I would like to do is that when a customer buys the samples kit for 3€, if he decides to buy the full size product on a next order he will get the 3€ refund.

How can I manage this?

Thanks Nicolas