Question about installing a lawn irrigation system while building a new home

Hope this question isn't out of place here, it was the best sub I could think of.. Say somebody is building a home and they have a landscape budget of $2,500 through the home builders they are working with, and they want to use this money to have a separate company install an irrigation system that costs $4,000 ..

What's the easiest way to make this transaction between new homeowner, home builder, and irrigation company?

The homeowner will obviously have to pay the difference of $1500, but does the home builder usually give the money to the homeowner so that they can pay the irrigation company in full, instead of making the irrigation company split the cost in two and having to accept the payment from two separate sources?

Thanks.