Why would the seller of the more valuable property reduce the price – is it just over the threshold?
If it is just over the threshold then you may be ok but if it is a lot over the threshold then the transaction may be looked at quite closely.
I used to work in conveyancing and people would try a similar thing by
saying the fixtures and fittings (which are not taking into account for stamp duty) were £30k and the house only say £200k or whatever they needed to get under the threshold.
Never worked and HMRC would be all over them like a bad case of herpes.
Seek professional advice and get the advice in writing. If it all goes tits up you can take it up with the adviser…