Most of the people selling LNG for instance, do not have any control over the definition of a "cubic meter". Even so, none of them cheat, because the US for example, very much does have its own definition of a cubic meter and it isn't going to pay you a penny more, nor a penny less, than what that cubic meter is worth.
All that to say, you could probably try to cheat the system, but I'd imagine the people in Sumer and Akkad had what they considered to be a precise unit of weight with which to measure your delivery. It doesn't matter what someone in Mohenjo-daro said, you were only going to get a certain amount in trade for your freight in Sumer. So I could see a centralized authority for weights, (the customer), at the same time as having no one in charge of that unit of weight in Mohenjo-daro.
I could see people agreeing to it essentially because that's all you're getting paid for. Because I saw the same behavior long ago at work with Halliburton.
The trade actually involved different measurements. The shekel (silver, commodity-money) was weighed by the Sumerian purchaser and then given to the trader in Dilmun (he would literally have a bag full of weights and silver)
The Meluhha commodities themselves were measured seemingly with the Meluhhan weights. So the units went from Indus -> Dilmun in Indus quantities, and were purchased and verified that way. The Sumerian guy was buying an "Indus quantity" and paying in a weight "Sumerian silver." So there wasn't a disconnection between Dilmun and Mohenjo-daro like you're implying.
It doesn't matter if you measure LNG in litres, gallons, or cubic inches. The market is going to pay you the precise amount for whatever you delivered measured in cubic meters. So what you measure in is irrelevant. Or more precisely, only relevant insofar as you want to have some idea how much you are going to be paid when you reach the market. And even then, it's only relevant to you. The market doesn't care what you measure in, because the market measures what you deliver in cubic meters. And the market has its own definition of "cubic meter".
So market participants, no matter where they may be, are incentivized to ensure whatever unit they're measuring out for delivery in will relate to the "cubic meter" as defined by the customer in a precise and verifiable way. If not, they could lose money. Thus, every freight measurement standard, would, again, effectively be set by the customer. Because no matter what units you deliver in, s/he is only paying you for the amount measured in his/her own measurement standard. So your standard has to conform to that standard in a fashion that is well defined, consistent, and well understood by you.
In ancient terms, this means the trader in Mohenjo-daro would have calculated that conversion factor out in a precise fashion. So they would walk around Mohendro-daro with a standard weight for measurement that was based on what they would expect to happen in Sumer. All the other traders in Mohendro-daro would eventually discover the same conversion factor. So everyone lands on the same weight for measurement, but no one collaborated. Because everyone, (every trader anyway), only cares what they can get for that weight in Sumer. It's not that everyone uses Sumer's units. It's that Sumer's units define what everyone's understanding of their own unit is through trade.
Most of the people selling LNG for instance, do not have any control over the definition of a "cubic meter". Even so, none of them cheat, because the US for example, very much does have its own definition of a cubic meter and it isn't going to pay you a penny more, nor a penny less, than what that cubic meter is worth.
All that to say, you could probably try to cheat the system, but I'd imagine the people in Sumer and Akkad had what they considered to be a precise unit of weight with which to measure your delivery. It doesn't matter what someone in Mohenjo-daro said, you were only going to get a certain amount in trade for your freight in Sumer. So I could see a centralized authority for weights, (the customer), at the same time as having no one in charge of that unit of weight in Mohenjo-daro.
I could see people agreeing to it essentially because that's all you're getting paid for. Because I saw the same behavior long ago at work with Halliburton.