Seeking Term and Conditions clarification

[#1]

Hello, I hope everyone is doing great ! My question is: I have developed a software few weeks back and i have seen a job where the clinet exactly wants what i developed. My software has only backend and the client wants a front end with my developed backend. basically i have to develop front end on top of my already developed backend. Does this consider as buying as selling and will it go againt upwork T&C?

thank you for helping me.