"Could" as a modal verb indicates a possibility, so it can be used in a forward-looking way as in your example:
How could we recover this
much amount from him in future if he will has not earned any income?
(I am not going to get into the other amendments I made because they are off-topic)
It isn't the use of the word "could" that is putting the sentence into the future tense though - the verb here is recover, and it would be in the future tense even if you said:
How will we recover this much...?
Modal verbs can be used in the past tense as well. For example, you might say:
When I was young I could stay up as late as I wanted.
This indicates that staying up late was a possibility for you at any time in the past.
Regarding your final sentence:
I could do it for you.
This is an offer to do something for someone. As "it" has not been done yet you are speaking about the future. But context could determine if the task is being done in the present, for example if you saw someone undertaking a task now you could offer to immediately take over that task by saying "I could do it for you".