笔 (bǐ) is a measure word used for sums of money, transactions, or deals. While 笔 literally means "pen," as a measure word it is used to count financial amounts, as in 一笔钱 (a sum of money). It can also be used for other business-related items like orders or accounts.
这笔钱你打算怎么花?
Zhè bǐ qián nǐ dǎsuàn zěnme huā?
How do you plan to spend this money?
公司最近谈成了一笔很大的生意。
Gōngsī zuìjìn tán chéng le yī bǐ hěn dà de shēngyì.
The company recently closed a very big deal.
他账户上有好几笔转账记录。
Tā zhànghù shàng yǒu hǎo jǐ bǐ zhuǎnzhàng jìlù.
There are several transfer records in his account.