可以说we are afraid that we can't return it吗？

可以，语法没问题

想要更委婉些的话可以can't 改成 couldn't

可以啊，已经是比较委婉的说法了。

We are afraid that we wouldn't be able to return it.

不可以说

be afraid to/of doing

没有be afraid +从句 这种用法。。