Explaining Assignment Fee to seller

[Joe Threats]

Is it necessary to tell your seller you will just put under contract and plan is to assign and flip this PSA? 

I was just thinking right before my seller signs I would mention to him I may assign this contract to another cash buyer. Is that fine with you? 

He asked for 22k negotiated to put under contract for 19k so I can have a little spread for my cash buyer. This is my first deal and I really want it to go through.