如果一間有限公司開了一張 ２１days PD Cheque 向另外公司買貨，到期日拿不到錢，可以嗎？合法嗎？可以怎做？

到期未能收到錢，當然不可以及是遺法，是欺騙罪，可以控告他，希望幫到你

In principle, not acceptable and may trigger claim for such default for payment and damages by law. Goods already delivered but payment withheld. This maybe an overlooking. I suggest you speak or better write to your customer for such overlooking and demand him to pay you by cash or to re-deposited this cheque again for immediate clearance. You could ask him to reimburse you the bank charges thereof. If still in default, you could send him a warning letter asking for immediate settlement or have to put this for legal action - small claim tribunal or for court action.