[AnthonyMouse]

>If the currency is deflating, even predictably, then whoever gets paid has an automatic advantage over whoever does the paying.

If the value of the currency is changing predictably and the date of the future payments is known then it can be taken into account in determining the amount of the payments. However, the purchaser does benefit if the seller doesn't know enough to account for it or is timid and fails to assert the demand, etc.