That $100 was supposed to go toward next month's bill here.

That $100 was supposed to go toward next month's bill here. (that one hundred dollars was supposed to go toward next month's bill here)

这句话可以理解为：

那100美元本来是要用来支付这里下个月的账单的。

重点：be supposed to

be supposed to + 动词
👉 应该……；本来应该……；按计划应该……

这里用了过去式：

was supposed to
= 本来应该…… / 原本是打算……

例如：

This money was supposed to pay the rent.
这笔钱本来是用来付房租的。

go toward

go toward something
👉 用于……；拿来支付……的一部分

所以：

go toward next month's bill
= 用于支付下个月的账单

例如：

This money will go toward my tuition.
这笔钱将用于支付我的学费。

The $50 will go toward the repair.
这50美元会用于支付维修费用。

整句拆解

That $100
= 那100美元

was supposed to
= 本来应该 / 原本打算

go toward
= 用于……

next month's bill
= 下个月的账单

here
= 这里 / 这边

所以非常自然地说：

那100美元本来是要留着付这里下个月的账单的。

这里的 was supposed to 很值得记住，因为它在美剧和日常口语中非常常见。