Change buy delay only on last bar

Dear all
I am a long time trader on a weekly base. I look for AB signals on weekend and invest on Monday with open price. So I made my backtester settings with "Buy price: Open" and "Buy delay: 1"
I have the problem, that I get the signal first on Monday when AB gets the price for Monday.
Or I made my settings with "Buy price: Close" and "Buy delay: 0". Then I get the signal on Friday, but I have a wrong backtesting with different buy signals.

What I need is an AB code to change the buy price and buy delay only for the last bar.

I tried a lot of variations, but nothing works.
Did anyone have the same problem and a solution?
Any suggestions?

Thanks from Germany
Mathias