[Avodah] paying in advance

[Eli Turkel]

<Immediately after I sent this, I realized what must be the reason why
you're not chayav if you pay in advance. R' Bodner doesn't say it, but
it must be that you're not chayav to pay for work until the work is
actually done (because there's still the possibility of the work being
cancelled or postponed to another day).>

This is what R. Zilberstein said. My question was on the rationale not
the "bare" halacha. Everyone agrees that one is not required to pay in
advance.
The "purpose" of the halacha is so that the worker need not wait for
his payment. As the pasuk says he is anxiously waiting for his payment
for his livelihood. If so it sounds strange that one loses the "aseh"
by paying in advance which is even better for the worker. Agreed this
might be a bad deal for the employer but if he volunteers to do
"lifnin mi-shurat hadin"" why should he lose his aseh?

Are there other cases where doing "lifnin mi-shurat hadin" costs a
person in terms of mitzvot?

-- 
Eli Turkel