I am not a lawyer, and I suggest you get one and ask this question of them.

When you have the lawyer write up the contract the best thing is to probably include a non-discloser agreement in it, that way they cannot disclose the workings of your code.

Next you will want the contract to say they are just licensig your code, they do not own it, they cannot change your code and resell it, nor can they reverse engineer it to re-create and sell it.

Those would be a couple of things to talk to the lawyer about. The lawyer will probably have more, the big thing is you want a contract that keeps this customer from taking your code and changing it to sell as something different.

Goog luck, and get that lawyer.