Hi
formula for laying to a fixed liability is:
liability/(lay_price -1) = bet amount
e.g. for a liability of 100:
100/(4.2 -1) = 31.25
so you could make a condition:
and selections trigger expression bet amount*(lay_price -1)is equal or less than 100
e.g. for bet amount of 50
50*(4.2 -1) = 160
the condition will return FALSE