This Torah thought is being dedicated to my beloved father, Nachman Shimon ben Yehuda Meir Hakohan, Z"L.
"Against all of the Children of Israel, no dog barked." (11:7)
When the Jews left Egypt during the redemption, not a single dog barked at them. As a reward for "holding their tongues", the Jews were commanded to feed a dog a "treifah" (a nonkosher animal by virtue of a wound). We find a similar concept in regard to a donkey in verse (13), "Every firstborn donkey you shall redeem with a lamb." Rashi comments that a donkey is given this special distinction because the donkeys carried the spoils of the Jews out of Egypt.
A question can be asked as follows: It seems that the dog is getting more of a substantial reward than the donkey! The dog is being given meat that it can enjoy in the present, while the donkey is only being redeemed by a lamb. This is a great 'honor" for the donkey in comparison to other nonkosher animals, but still would not the donkey rather receive a reward that it can enjoy on a more materialistic level like the dog's reward.
I would like to, Bezrat Hashem, propose an original answer that perhaps can teach us a very valuable lesson. By nature a dog is inclined to bark at any occurrence. Yet, the dogs of Egypt went against their very nature for the sake G-d. A very great and special event was happening in Egypt when G-d was redeeming the Jews from Egypt, it would not have been honorable to have dogs barking, they therefore went against their nature and " held their tongues". However, the donkey is inclined to carry huge loads on their backs, this is their nature.
The donkeys really did not do anything more than their natural inclination, namely to be carriers. They are still being rewarded for their good work, yet the reward is some what less than the dogs. It takes real effort to go against ones natural tendencies. Therefore, the dogs reward is much more substantial materially than the donkeys.
Every person is born with certain natural tendencies towards sin. But the reward is greater when a person can overcome these inclinations , and stop themselves from sinning for the sake of G-d's glory. This takes much strength and effort.
This is precisely what the Sages meant when they said "Mishom tzara Agra", according to the pain (effort) is the reward. Have a good Shabbos