How can one repent from breaking a covenant through deception, while continuing to deceive in other matters? And will Allah accept repentance for breaking a covenant without repenting from deception? And what should be done with the sustenance obtained because of that?

The answer can be summarized in the following points:

Firstly: Whoever repents from one sin but not another, their repentance is valid for the sin they repented from, but the sin they did not repent from remains upon them. Secondly: Whoever made a covenant with Allah and broke it, must offer an expiation for a broken oath (kaffarat yameen). Thirdly: An expiation is due for every covenant made with Allah that was broken. The covenant is dissolved by breaking it, and no further expiation is required unless the covenant is repeated. Fourthly: There is no need to get rid of something Allah has blessed you with; rather, you must use it in ways that please Allah. Fifthly: Sincere repentance (tawbah (definition) nasuh) from all sins, including cheating, is obligatory. This entails immediate cessation, firm resolve not to return to it, and remorse.