n00b question: what is this 50,000 yen difference?

[CluelessToshika]

So, my n00b self purchased 1,000,000 yen's worth of "三菱ＵＦＪ－ｅＭＡＸＩＳ　Ｓｌｉｍ　米国株式（Ｓ＆Ｐ５００）" via my shiny new SBI account.

Today SBI tells me this has now become 1,021,720 yen (w00t party time).

Now, if I click on the 売却 link (don't worry, I'm not actually planning to sell them, just trying to work out how this stuff works), under "注文可能金額" it shows 970,634円, which by my calculations is 51,086 yen less than 1,021,720.

I can't upload images here, but here's a screenshot of the relevant bits to make it clearer what I'm talking about: https://imgur.com/a/JDaNG7g

What is this 970,634円 figure?

[CluelessToshika]

OK, googled it: 投資信託の売却画面に表示される注文可能額が少ない理由を教えてください.

So it's 5% less than the 基準価額 as a buffer in case the price changes when the sale is actually executed.

If I wanted to be sure to get rid of the entire lot, presumably I'd enter the 口数, or just select 全額.

(Sorry for the noise, thinking out loud here).

[banders]

This was an annoyance when I was trying to sell and rebuy in new NISA in December. I forget the exact details but it was something like I sold 80 Apple shares hoping to immediately rebuy the same amount, but couldn't, even at the same price, because of that buffer. By the time the sale had been processed and the buffer released, the price had risen and I was left with 78 shares total.