H1B transfer to the current employer


I am currently working for the company ABC (big firm) and company XYZ(small firm) filed my H1B petition (2014) and the petition is approved by USCIS. Now I will have to go for stamping to the US consulate.

Inorder to avoid the risk of stamping (since XYZ is a small firm) I am planning to go for H1B transfer (under CAP Exempt) to my current employer ABC. My current employer ABC has agreed for the transfer of my H1B under CAP exempt but asked to provide the documents like i797 copies and Perm, I140(If applicable) from my petitioner company(XYZ).

But my H1B petioner(XYZ) is not willing me to provide those copies. I have read in some Immigration blog that I can get these copies from USCIS by submitting a Freedom Of Information Request(FOIR). Is that true? If yes, I can get the copies from USCIS in 3 months.

Is there something like I should be transferred to the employer(ABC) before October 1st? Or I can be transferred later also, in which case do I need to submit any payslips from the petitioner company?

I will be very thankful if you could help me on this.