Were all US soldiers paid in full during World War One (WWI)? Were the Bonus Wars not actually asking for additional money but for back payments for salary never paid?
I think you're referring to the Bonus Army. WWI vets were paid in full. The issue was over, you guessed it, a bonus for serving in a war zone.
Traditionally, US soldiers received a bonus for serving during war-time to make up the difference between what they would have earned as a civilian. This was not done after the Spanish-American War, and after WWI they got only a $60 bonus.
In 1924 the World War Adjusted Compensation Act attempted to correct this by offering a bonus of $1 for every day of the war served in the US, and $1.25 for every day overseas up to $625. President Coolidge vetoed the bill saying "Patriotism which is bought and paid for is not patriotism." Congress, realizing you can't pay the bills with patriotism, overrode the veto.
Up to $50 per veteran was to be handed out immediately. The rest would go into an Adjusted Service Certificate to be paid out, with interest, in 20 years. They were basically given a government bond redeemable in 1945. Vets could use the bond as collateral to borrow money, but they couldn't cash it in until 1945.
Then the Great Depression hit and suddenly these veterans were out of work and needed money right now, not in 1945. The Bonus Army, and others, demanded to be able to redeem their Certificates immediately. After a Congressional bill to do just that failed in the Senate 62 to 18, the Bonus Army was forcefully evicted by the US Army.
Finally, in 1936, Congress passed the Adjusted Compensation Payment Act which replaced the old Certificates with normal Treasury Bonds at an interest of 3%, better than a savings account, which matured in 1945. The vets could redeem them before then for cash, and most did. The sudden influx of $1.5 billion into consumer hands provided a quick, easy economic stimulus to counter the ongoing Depression.
At the end of WWII, rather than simply give a bonus and send the vets on their way, the issue of what to do with returning soldiers was handled with the Servicemen's Readjustment Act of 1944 aka the G.I. Bill that the US still uses today. This includes veterans benefits for unemployment, school, home ownership, and starting a business in effect helping to retrain millions of soldiers for civilian life.