Minecraft java command block reply "you don't have enough money"

Use /scoreboard players test to check if a player has a high enough score. You'll want to place the test command in an impulse block, then have your current impulse block be a chain pointing to it. Then, place another impulse block to test if the player has less than 100 money, and place a chain block behind it that displays your 'not enough money' message. Connect both the impulse blocks to your redstone circuit so that the test to see if they don't have enough money triggers first. Otherwise, it may complete the transaction then tell the player they don't have enough money because they spent it.

Use /execute if score:

execute if score @p money matches ..99 run tellraw @p "currency deficiency"
execute if score @p money matches 100.. run tellraw @p "purchase made"

Replace the /tellraws with whatever you need to make your machine work.

I also see that if you have enough money, you're going to want to run multiple commands if the condition passes. Your can either have one check per step:

execute if score @p money matches 100.. run tellraw @p "purchase made"
execute if score @p money matches 100.. run scoreboard players remove @p money 100

Or you can setup a function.