In 17:23 the word qada has different shades of meanings depending on context, going from setting limits to measuring those limits to acting according to those limits. See 6:91,12:68,28:15,28,33:23 etc. The word fits 17:23 in the sense that Allah has set limits to the humans in regards to worship which they are obligated to fulfill, just like any citizen under the rule of law. Fulfilling that law does not entail through compulsion, but could equally be through freewill. This is clear in 33:36 where disobedience to Allah's qada/decree through His messenger is forbidden, although it is possible
"and whoever disobeys Allah and His Apostle, he surely strays off a manifest straying".
In 17:4 Allah qada/decreed that the Israelites
"will make mischief in the land twice, and most certainly you will behave insolently with great insolence".
However that decreed disobedience only came about through God's foreknowledge of their own will
17:7"If you act rightly, it is for your own good, but if you do wrong, it is to your own loss".
The possibility was there for them to act good and the decree to be otherwise. But God knew that in their freewill they would choose evil instead.
So Qada entails obligation, although not devoid of freewill, while the very similar Wassa is softer in tone. It entails trusting a matter, hence its use in Arabic for the will of the deceased or anything of importance one asks another to take care of. Both words can be used interchangeably to convey a particular nuance depending on the general tone of the context. The use of wassa in 4:131 creates a softer tone. Had qada been used, grammatically it would have to be followed by aala or ila as in 17:4, and the general feel of the verse would have been different, more forceful. Similarly the use of wassa for the revelation 42:13 is more fitting as it is speaking of it being entrusted to the prophets, the devoted slaves of God who do not need to be "obligated" with it.
Further reading answering The Islam Issue "Ibn Abbas testifies to Quranic corruption"
- Islam Critiqued cannot ask with humility...(variants 2:214,18:79-80,24:27,17:23,4:24)
- Islam critiqued wakes up with the wrong foot..(variant 2:198)
- Sam Shamoun "Muhammad and the Ten Commandments" (mutah and 4:24)
