That sounds OK to me. I'd say "The decline in sales" - not 'amount'. And maybe "our customers are experiencing ...". But I wouldn't call your sentence unnatural. Maybe your boss thinks it should be "lack of finance", or "lack of money" to put it bluntly, but "a lack of financing" means "an inability to arrange the necessary finance/money" not merely a lack of money.The decline in sales amount can be explained by the fact that our customers experience a lack of financing. Therefore they tend to choose cheaper equipment.
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