If the boots are letting in significant amounts of moisture in such a short time, then possibly they may have a manufacturing flaw.
There is no such thing as a fully waterproof boot, some boots are made to a higher specification than others, and are more watertight, but either way, a leather or fabric boot will eventually get damp after a certain time exposed to moisture.
Your issue sounds more promising, as a walk in damp grass should not instantly let in water inside the boot, leather of fabric.
There is no harm in returning them to the shop, and voicing your concerns, the boots may indeed have an issue, and it's best to draw the shop to a possible problem,sooner than later.
Do not be badgered or told by the staff that there is nothing wrong with the boots, if your not 100% happy with your purchase, then return them.
You may not receive a full refund since the boots have been worn outside, but you may get a sizeable discount on a more suitable pair.
Boots are manufactured to exacting standards, and occasionally some leave the factory with a defect, it's rare, but it can happen.