How would you manage a team in your project if you are a Project Manager

Question

Suppose you are managing a new software development project and one developer joins your team who was previously working on another development project.

But still his ex project manager asks his help on the old project. Because of this, he is not able to focus on the current project.

What should you do in such a situation?

Answer 1

I would ensure that before a new team member joins my project, I'd want to get some level of commitment from the functional manager regarding that team member's availability. 

Just to be sure, I will also likely ask a few questions to sanity check the allocation amount. 

After this, if I find that there is a variation in actuals relative to expected, I would first check with the team member and then with their functional manager and if this variation was likely to be more than for a day or two, it would be managed as a change.