But what's wrong with:
'Hey X, how are you? Listen, I know this is incredibly short notice and you're probably busy but are you free today as i need a hand with x,y,z? ...... You're not, no bother mate, thanks anyway! How are Summer and the kids, you're all still coming up on X day right? Bye!'
There, done, conversation over in 2 minutes and nobody feels irritated or abused.
As opposed to what said friend did to us last year:
'Hey X how's things? [10 minutes of chat] So what you up to on Friday? [DP answers 'nothing much' as he thinks friend wants to call round, they're musicians and work on projects together] Oh, that's great... [the missus] and I were hoping ds could stay the night at yours, we want to go out for a meal. I'd drop him round about 4 as we have a table booked for 8 [they live 1.5 hours away]... that'd be ok right?'
a) the child doesn't know us very well
b) it was the night before valantines day so that would have meant having a child we don't know well in our house for most of valantines day... and maybe we would have wanted to go out too? 
c) dp might have been free but the kids and i weren't and it would have meant dragging the child to dancing and other activities and having him in the same bedroom as our 12 month old who didn't sleep well.
d) the assumption that just because dp was free that we'd be willing to do it, it's not like the favour would be returned as we have 3 kids, one much younger than theirs.
dp felt cornered into saying yes, told me and i hit the roof and had to ring him the next day and say 'Actually, Summer has just reminded me we were supposed to be going to her parents that night, sorry about that'
Both parties ended up pissed off as a result when a simple polite request would have saved all the hassle, instead fo backing dp into a corner.