Decrement Total In One Table Based On Value In Another

I have an procedure that uses a cursor to decrement a total in one table based on value in another. I am trying to rewrite this to be set based but having some issues getting the correct values.

one table will have a total amount available and the other table will have line items for the amount to decrement (think amount left on gift card). If the total amount remaining is less than the line item amount then line item amount should be set to total amount available with total amount set to zero.

this last part is where I am having trouble.

Here is example table

CREATE TABLE qamt(id INT, amt INT);
CREATE TABLE qtot(id INT, tot INT);

INSERT INTO qamt
VALUES(1,10),
(1,20),
(1,30),
(2,90),

[Code] ....

In the end I would want to update the qamt and qtot tables. I basically want to accomplish a quirky update across multiple tables.