Dear,

1. Well, does the rental income credited into your account or the owner's account? If it's the owner's account, definitely it isn't yours. Hence, you can't use other people's supporting income to increase your loan chances right?

2. Your question is quite confusing, you said seller gonna rent back to the new owner, hence you haven't purchase yet, where will he get that rental agreement?

CHeers

As for now the bank don't look at this way. Otherwise everyone would use this against the bank system and over borrow as they would ride on the rental as extra income which is unrealised yet.

The bank cannot take in uncertainty

Sale and leaseback is common but happen more frequent in corporate segment.  The lease / tenancy agreement does add value but you still need to satisfy the bank that without that you would still be able to service the loan as there are uncertainties as to the credibility of the tenant (which is the seller) and continuity of the tenancy.  Even for corporate segment these risks are taken into account too.