Journal entry scenario clarification.

Hello Sir.

I have a doubt regarding the journal entries in the following scenario :

A purchases goods worth 100 from B. The payment is made on the same day. On receipt of the goods, A notices that goods worth only 80 have been sent by B. On discussion, B acknowledges the mistake and promises to pay back the remaining amount (20) at a later date. The remaining amount is eventually received at a later date.

The journal entries for this scenario, according to me, would be :

In the books of A –

1. purchases a/c dr. – 100
to bank a/c – 100
2. receivable a/c dr. – 20
to purchases a/c – 20 (doubtful about this credit)
3. Bank a/c dr. 20
to receivable a/c – 20

In the books of B –

1. bank a/c dr. – 100
to sales a/c – 100

2. sales a/c (or sales return?) dr. – 20
to payable a/c – 20
3. payable a/c dr. – 20
to bank a/c 20

This is my understanding of the concept in this scenario, but I’m doubtful. Hope you can clear this up for me, Sir. Thank you.