Giving everything (or even 10%) is an evangelical counsel: it's not required for salvation. As you quote, the early Church only directed its members to give what they could, not a minimum amount (much less everything). Paul even writes, just before the passage you quote, in 2 Corinthians 5:5:
So I thought it necessary to encourage the brothers to go on ahead to you and arrange in advance for your promised gift, so that in this way it might be ready as a bountiful gift and not as an exaction.
And while I'm no Biblical scholar, Matthew 19:21 ought to be read in context. Just preceding this verse is Matthew 19:16-17:
16Now someone approached him and said, “Teacher, what good must I do to gain eternal life?”
17 He answered him, “Why do you ask me about the good? There is only One who is good. If you wish to enter into life, keep the commandments.”
That's it: the only thing required for salvation is to keep God's commandments, which Jesus enumerates in Matthew 19:18-19: poverty or charity is not listed among them.
Then taking the verses after and including Matthew 19:21:
21Jesus said to him, “If you wish to be perfect, go, sell what you have and give to [the] poor, and you will have treasure in heaven. Then come, follow me.”
22When the young man heard this statement, he went away sad, for he had many possessions.
23Then Jesus said to his disciples, “Amen, I say to you, it will be hard for one who is rich to enter the kingdom of heaven.
24Again I say to you, it is easier for a camel to pass through the eye of a needle than for one who is rich to enter the kingdom of God.”
25When the disciples heard this, they were greatly astonished and said, “Who then can be saved?”
26Jesus looked at them and said, “For human beings this is impossible, but for God all things are possible.”
Jesus is saying in verse 21 that if you wish to be perfect, you ought to sell everything and give to the poor, but the verses that proceed from verse 21 explain how impossible that is for people without God.
The disciples' astonishment and query to Jesus in verse 25 speaks to this, essentially saying, "whoa, nobody's going to do that: does this mean nobody will be saved?" Jesus in verse 26 then specifically states that God will save human beings by performing the impossible task: saving those who couldn't normally be saved because they aren't perfect.
Or as the New American Bible explains:
Riches are an obstacle to entering the kingdom that cannot be overcome by human power. The comparison with the impossibility of a camel’s passing through the eye of a needle should not be mitigated by such suppositions as that the eye of a needle means a low or narrow gate.
Beyond that, the Didache, an early Christian writing that dates to around the same time as Paul, mentions one of the ways you could tell a false Apostle from a real one:
Let every apostle that comes to you be received as the Lord. But he shall not remain except one day; but if there be need, also the next; but if he remain three days, he is a false prophet. And when the apostle goes away, let him take nothing but bread until he lodges; but if he ask money, he is a false prophet.
It wasn't until much later that justifications for tithing were formulated: the Catholic Encyclopedia mentions the first serious attempts to tithe dated to around 567 CE and 585 CE with the Councils of Tours and the Maçon, respectively.