Please help

[Zoe2008]

Ms. Leung has requested payment last week, but we are busy to tidy the document for the coming ISO audit 02 April. So that did not arrange it to them, but have explained to her that it will be settled by this week. Don’t worry, we will not hold their payment.

[LeighS]

From what I've understood, you could rewrite it like this;

Ms. Leung requested payment last week, but we (have been/ were) busy organising the document for the upcoming ISO audit on 2 April. So it was not possible to arrange it then, but we have explained to her that it will be settled this week. Don’t worry, we will not withhold their payment.