Since lying is disrespect, do nothing. Don't reply, don't indulge the excuse. Don't say "Okay, maybe later." She's disrespected you and must make effort to regain your favor. If she likes you, she will contact you again soon. If not, it was basically done anyway. Chasing after an obvious lie only weakens your position and makes you look desperate. The mindset of saving it comes from scarcity, where you're hooked and scared to lose it. By not caring, you tell yourself she lost out.