Hello comunity

I would like to know if i can use an alias with the sum function.

Suppose i have several client orders like This:

Article quantity requested. Quantity in invoice. rest.
B001 30. 10. 20.
B001. 25. 0. 25

Now i introduce my supplier order request
Article. Qtt
B001. 30

I make the scan for my clients order and i must check the quantity That i can satisfy with my supplier order.
Then for the for the first line of my client order, i can satisfy 20 (30-20) the rest on supplier order is equal to 10.
I Need to keep the value of 10, because for the line 2 of my client order the value is 25, then i can only satisfy the value of quantity 10.

If i use :
Sum qtt into my cursor crssupplier

I lost the value of 10 because the scan have move the focus the the secund line of my client orders.

Is it possible to use the value stored in my cursor crssupplier with an aliás?, in This case when the cursor move to the line 2 of my client order, the value of rest must be equal to 15.

Many thanks,
Best regards
Luis santos