I came across the following sentence:
How 働かせ is understood in this context? It looks like an imperative form of 働かす, but the imperative form over here doesn't make any sense here.
働かせ can be either the continuative form of 働かせる or the imperative form of 働かす. Weblio
The 働かせ in your example is the former. You can rephrase the sentence as 「4000万円までは借金を膨らませて働かせて、5000万円になると～～」.
For more about using 連用形 as a conjunction, you could refer to: