all is fair in love an war. and sales.

Can't beat the master at her own game. What are you thinking man!

She's not a master, just a cheater. "Stealing" sales means he is doing all the work and she is simply getting the credit. She's abusing her position as manager to steal money from him, she should be fired for that.

OP, your manager is a bitch

How could they be your sales if she closed them? Doesn't the commission go to the closer? So start closing faster. When she steals a customer that you started with, change course and target her customers. Take enough of hers and she'll know not to mess with you.

She's his manager. He has no choice. She's in a position of power over him, so he has no way to retaliate other than quit.

Well now it's a challenge

This needs a follow up be

I had a manager like that once. It didn't *really* matter because we didn't make commission. But damn was it annoying to see her bragging about her high sales every month.

Hope she bought you a beer afterwards to apologise