I tend to use the following construction often in my e-mails:

It would be great if you could...

However my German colleague repeatedly reminds me that according to the rules he was taught, using "if" together with "could" in the way I do is strictly speaking incorrect. I am however certain that this construction is quite common (see here) and can often convey willingness rather than ability.

Is the above construction strictly speaking incorrect, and is there a better replacement for it?

I tend to use the following construction often in my e-mails:

It would be great if you could...

However my German colleague repeatedly reminds me that according to the rules he was taught, using "if" together with "could" in the way I do is strictly speaking incorrect. I am however certain that this construction is quite common (see here) and can often convey willingness rather than ability.

Is the above construction strictly speaking incorrect, and is there a better replacement for it?