How to get the best offer from vendors although you have some financial obstacles in settlement on cash basis?

It can be written a book on this subject. Some principles are, in my opinion, to be taken into consideration:

1. It's always good to have as many alternative suppliers as you can. They will compete and push the price somewhere were it is reasonable. This does not necessarily work in high tech supplies, such as very specific equipment, where, if you decided the engineering constraints, the supplier's area becomes extremely narrow.

2. If you have cash flow problems, you have the options to supplement the cash input (banks, business angels, investment funds, even selling some actives or shares of your present business), or, either, to delay the project. Or, to negotiate longer payment delays from the very beginning. However, in the last case, the supplier will usually add to the price the bank interest for the specific amount and delay. It is  definitely not an option to delay payments more than negotiated ones, because such a behaviour cannot remain unknown and suppliers will start asking for safe payment instruments. Exists, also, another practice, to claim quality issues and further delay payment, which is definitely unfair and , from the PR point of view, a disaster. I hope this touches the sensitive aspects you needed to be covered.