H1b Transfer and travel plans

[Goluram]

Currently working with Emp A, H1B and visa stamp is valid thru early 2018. Got an offer from Emp B who is ready to transfer my H1B under premium processing and wants me to start from 1st week of July. I have some urgent plan to travel out of US during 25th of June, my question is while coming back can I use Emp A's I-797 at POE or do I have to use Emp B's I-797 (given the fact Emp B receives my Approvals before that)? Since I am starting with Emp B in July first week will there be any issue?