After True Pacifist route, you chose to stay with Toriel... Although somehow Asriel Dreemurr got revived, you knew that and had started to be great friends and welp after some time...

Sans teleported into Toriel home and noticed Asriel but he didn't looked up and thought it was Toriel since Asriel's clothes were not much different from Toriel's as he start to... welp...

Sams got quite nervous as he held Asriel's hand

Sans: Hey, Tori.

Asriel: !

Sans's eye lights looked away for a moment.

Sans: You know... We should hang out more often.

Sans then closed his eyes as his grin widens

Sans: I wanna get to know you better, be closer to you.

He opens his eyes with a wiggling grin as blue blush appears on his bone-cheeks

Sans: What I'm trying to say is... Wanna go on a date

Sans then looks up and his expression turns into confusion then into shock

Sans: ... with me?

?????????

Sans realizes as his chest straighten, his left eye flashes blue and yellow from shock and panic, while right eye is still there, his SOUL skipped beat as he feels shocked, embarrassed, flustered

Sans: OH... UH...i thought you were Toriel.

As Asriel is left speechless, his cheeks flashes with pink blush as he also gets shocked and confused and flustered

You on other hand were watching all this from your room almost giggling, and you accidentally which made you laugh and welp... It caught both of their attention... Asriel on hand didn't feel anger but Sans... you feel like you're gonna have bad time