Ifelse condition over a mask

I have a problem. Imagine I have a data set a:

     row1 row2 row3
col1  2    3    5
col2  5    3    4
col3  3    1    6

And I have a mask, which identifies those entries, which should be transformated:

     row1 row2 row3
col1  T    F    F
col2  F    F    T
col3  F    T    F

So basically, I want all of the entries which are labelled as TRUE (T) be replaced by their value they have right now minus the value out of another dataset b:

     row1 row2 row3
col1  1    4    8
col2  4    1    1
col3  6    2    7

So the result should be:

     row1 row2 row3
col1  1    3    5
col2  5    3    3
col3  3   -1    6

What i tried was:

new_dataset <- ifelse(Mask == 'FALSE', a, a - b)

However, I end up with a list instead of a data frame. I know, that is because R creates every entry within the list by the total dataset a or (a-b). But how can I handle this?

Thank you very much in advance! :)