Business plan is a rough calculation - do the numbers stack up, is the project worth doing: "Should we do it"
Feasibility plan is more like a PID - it's a detailed plan, ending up with who will do what and when. So you can look at resources and see if you have the right people at the right time, and also look at
* the total time required - can the project be done by the required delivery date
* the total cost of all the tasks - can we do it for the money
If not then maybe the quality has to be reduced...
Therefore the sequence goes
Business Case - is ti worth doing a detailed plan?
If yes, do a detailed plan
Take it back to the stakeholders and either say Yes I can do it or "Sorry, it's not feasible, but here are some options. Would you like me to proceed with any of these?
Never say yes to a project without having planned it out first. Otherwise, if you fail, it's your fault, even if the project was impossible.